In U.S. Pat. No. 4,970,467, issued Nov. 13, 1990, entitled "APPARATUS AND METHOD FOR PULSE PROPAGATION ANALYSIS OF A PIPELINE OF THE LIKE", the inventor of that patent being the same as the applicant in the present application, there was disclosed a system for ascertaining the existence and location of anomalies along the length of a member, such as a underground pipeline. In that system, electrical pulses are imparted to the pipeline at opposite ends of the portion to be tested, with these pulses being synchronized so that they intersect at predetermined locations along the length of the pipeline. The waveform or "fingerprint" from one of the pulses which has passed through the point of intersection is analyzed to determine the possibility of an anomaly being present at the location of the intersection.
The particular problem toward which U.S. Pat. No. 4,970,467 was directed was to detect faults in pipelines which carry oil or some other fluid underground and extend for possibly hundreds of miles.
Such pipelines are commonly made of metal (e.g., steel) which is wrapped with a protective layer of tape to prevent corrosion of the metal. Even so, the protective layer will sometimes deteriorate at certain locations, or possibly be abraded by some object (e.g., a rock which might come in contact with the protective layer) so as to expose the metal of the pipe to the adjacent ground, resulting in premature pipe corrosion.
In order to alleviate this corrosion of the pipeline, it is common to utilize a source of electrical direct current power to impart a negative charge to the pipeline relative to the adjacent ground. One method is to attach Galvanic anodes to the pipe (e.g. a magnesium anode). Another method is to provide a DC generator with the negative output being attached to the pipeline, while the positive output is connected to an electrode which is placed in the ground. However, this has its shortcomings. For example, there can be a localized interfering electrical field which may reverse the electrical potential between the pipeline and the ground within an area. This electrical field could result, for example, from an adjacent pipeline which might cross (or extend adjacent to) another pipeline.
Accordingly, the pipeline industry has undertaken to analyze the conditions along the length of the various pipelines to determine the electrical potential between the pipeline and the adjacent ground. The common method of doing this is what is termed the "half-cell" process, which has more or less become the standard of the industry. A typical half-cell comprises a containing member which is a sealed plastic cylinder with a porous ceramic plug. A solution of copper sulfate is in the container and there is a piece of copper which extends into the solution of copper sulfate, with this copper being in turn attached to a wire which is then attached to a volt meter. The other lead of the volt meter would lead to a connection to the actual pipe itself. A somewhat crude method of taking half-cell readings would be to walk along the length of the pipe, dig a hole at selected locations to expose the pipe, attach one electrode to the pipe, and then stick the half-cell in the ground at that location to take a reading. Then the person would proceed to the next location along the pipeline and repeat the same process. However, there are more effective methods of accomplishing this. One method is to connect one end of a cable to the pipe at one location, and have the length of the cable wound on a rotating drum which is in turn mounted to a truck. The truck is then driven down the length of the pipeline for a few miles, with the half-cell being placed in the ground at various locations along the length of the pipeline.
When one realizes that pipelines extend beneath freeways, underneath rivers, underneath the ocean floor, and through other areas of difficult access, it can be seen that there are practical problems in employing the half-cell method. Nevertheless, the half-cell method has in a sense become the standard of the industry, and substantial work has been done in analyzing the data gathered through the half-cell method and correlating this to the condition of pipelines in the soil. The net effect is that there has been for many years a growing problem of substantial magnitude in effective detection of pipeline defects. In the United States alone, there is a vast network of pipelines extending along various routes, and there are conferences held between the various owners/operators of such pipelines to resolve the problems associated with these pipelines (e.g., the electrical field of one pipeline affecting another pipeline adversely). Also, the increasing sensitivity to environmental considerations associated with pipeline leaks is of greater concern. Further, the economic considerations of proper maintenance and functioning of these pipelines is significant.
Somewhat similar problems exist with regard to analyzing well casings, such as oil well casings which can extend thousands of feet downwardly into the earth. However, the analyzing of a well casing can be even more difficult, because of the inaccessibility of that portion of the well casing at greater depths below the earth's surface. In fact, to the best knowledge of the applicant herein, there has not been a reasonably adequate method for analyzing well casings or structures such as well casings where there are substantial problems in accessibility.
One prior art approach of analyzing the condition of various objects is time domain reflectometry, where a pulse is transmitted along the length of the member being tested, and at the location of a discontinuity, there is a reflection of the pulse which is sent back to a receiving location (which can be the location at which the pulse was transmitted). By measuring the time increment from the transmission of the pulse to the time the reflection is received, while knowing the velocity of the pulse, the location of the discontinuity can be ascertained. Also, depending on the circumstances, the character of the reflected pulse may yield information about the nature of the discontinuity. While this method has value for certain applications, to the best knowledge of the applicant, this has not proven to be an effective method of analyzing the condition of pipelines or wells.